You may find easily your accomodation in Beppu, where many hotels exist,
since it is a fmous hot spring place.
But soonest reservation is recommended as autumn is a best season for traveling in Japan.
The Kamenoi Hotel (https://kamenoi-hotels.com/en/beppu/) may be recommended, where the banquet is held.
For other hotels, please find a room by yourself through the internet.